1. Field of the Invention
The invention relates to a method of determining a magnetic resonance (MR) distribution in a part of a body using an arrangement of multiple surface coils, in which method by means of separate surface coils component distributions are determined and said component distributions are combined to form said NMR distribution.
The invention relates also to a magnetic resonance (MR) imaging apparatus for determining a MR distribution in a part of a body using such method, comprising input terminals for an arrangement of multiple surface coils, means to determine component distributions using separate surface coils of said arrangement and means to combine said component distributions to form a MR distribution.
2. Description of the Related Art
When acquiring MR images with several receiver coils simultaneously, sometimes called the synenergy concept, there is the problem to combine the separate results into a homogeneous result image.
Formally one can write that, using N coils, N images or volumes EQU s.sub.j (x,y,z) (Eq. 1)
are acquired. j.epsilon.{1, . . . N} is the coil index and S.sub.j (x,y,z) are complex voxel values at position (x,y,z). The problem is to combine these images into a single image (or volume) I(x,y,z) that is both optimal in signal-to-noise ratio (SNR) and homogeneous, i.e. that the same spin magnetisation density results in the same pixel value in I(x,y,z).
A method pertaining to the field of the present application is known from the European patent application EP 0 472 390 A2. In this patent application is described that it was usual to reconstruct composite images from MR phased arrays in the time domain by combining the individual image contributions on a weighted, point by point basis after first acquiring the complete MR images for each separate coil. The reason for acquiring the separate images first is that that the optimum set of weights needed to maximize SNR when combining the separate signals is a function of position, and so varies from point to point.
In EP 0 472 390 it is suggested to combine the signals in the time domain by convolving them with a time domain representation of a field map of the respective one of the receiver coils. However, such reconstruction requires the exact knowledge of coil sensitivity at each location.
In the article in Magnetic Resonance in Medicine, 16(2), pp. 195-225 (1990) is described the sum-of-squares method, where I(x,y,z) is calculated by: ##EQU1##
However, this result tends to be very inhomogeneous.
Several solutions for this problem have been proposed, but many of these tend to be relatively impractical and/or limited in the ability to correct for the inhomogeneity.
One possibility is to estimate the sensitivity of the separate coils by low-pass filtering (i.e. "smoothing" or "averaging") of the surface coil image itself. See, as an example: SMRM 1991, abstract 744, or the equivalents thereof for simple surface coils: SMRM 1987, abstract 266, and the article in American Journal of Radiology 147, pp. 379-382 (1986). The effectiveness of these methods is limited, particularly if the actual spin density in the object shows considerable a-symmetries.
It has also been suggested to measure a homogeneous water phantom with the same coil setup and to use the results for calibration. Yet, this method is very impractical.
Another approach would be to measure the same object with an additional, very homogeneous coil, and to use the resulting image to calibrate the intensities of the image obtained with the arrangement of multiple surface coils. In the European patent application EP 0 271 123 A1 such an approach is described for calibration of a single surface coil with the signals obtained by means of a body coil. However, this approach either costs additional scan time (when acquiring the homogeneous image before or after the actual acquisition with the surface coil(s)), or it poses a yet unresolved problem of simultaneous aquisition.
Other background art on synthesizing the separate images of surface coil arrays is disclosed in the European patent application EP 0 412 824 A2 and the U.S. Pat. No. 4,825,162.